Structures and Classes CS 16: Solving Problems with Computers I Lecture #15

Transcription

1 Structures and Classes CS 16: Solving Problems with Computers I Lecture #15 Ziad Matni Dept. of Computer Science, UCSB

2 WHAT THE NEXT 3 WEEKS LOOK LIKE MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY 20- Nov 21- Nov 22- Nov 23- Nov 24- Nov Lab 8 issued Lecture: Dynamic Arrays, Makefiles 27- Nov 28- Nov 29- Nov 30- Nov 1- Dec Lab 9 issued Lecture: Structures and Classes Lab8 due Lecture: Linked Lists Hw 8 due Hw 9 issued 4- Dec 5- Dec 6- Dec 7- Dec 8- Dec Lab aiendance is opjonal Lecture: Recursion, Search/Sort Lab 9 due Lecture: Review for Final Exam Hw 9 due 11- Dec 12- Dec 13- Dec 14- Dec 15- Dec FINAL EXAM, 4-7 PM 11/29/17 Matni, CS16, Fa17 2

3 Structures (Ch. 10.1) Defining structures Member variables and funcgons Structures in funcgons Hierarchy in structures IniGalizing structures Lecture Outline Classes (Ch. 10.2) Defining member funcgons and the :: operator Public vs. Private members Constructors 11/29/17 Matni, CS16, Fa17 3

4 What Is a Class? A class is a data type whose variables are called objects Some pre- defined data types you have used are: int, char, double Some pre- defined classes you have used are: ifstream, string, vector You can also define your own classes as well 11/29/17 Matni, CS16, Fa17 4

5 Class DefiniGons To define a class, we need to Describe the kinds of values the variable can hold Numbers? Characters? Both? Something else? Describe the member funcjons What can we do with these values? We will start by defining structures as a first step toward defining classes 11/29/17 Matni, CS16, Fa17 5

6 STRUCTURES 11/29/17 Matni, CS16, Fa17 6

7 Structures A structure s use can be viewed as an object Let s say it does not contain any member funcgons (for now ) It does contain mulgple values of possibly different types We ll call these member variables 11/29/17 Matni, CS16, Fa17 7

8 Structures These mulgple values are logically related to one another and come together as a single item Examples: A bank CerGficate of Deposit (CD) which has the following values: a balance an interest rate a term (how many months to maturity) A student record which has the following values: the student s ID number the student s last name the student s first name the student s GPA What kind of values should these be?! What kind of values should these be?! 11/29/17 Matni, CS16, Fa17 8

9 The CD Structure Example: DefiniGon The CerGficate of Deposit structure can be defined as struct CDAccount { double balance; // a dollar amount double interest_rate; // a percentage int term; // a term amount in months } ; Remember this semicolon! Keyword struct begins a structure definigon CDAccount is the structure tag this is the structure s type Member names are iden1fiers declared in the braces 11/29/17 Matni, CS16, Fa17 9

10 Using the Structure Structure definigon should be placed outside any funcgon definigon Including outside of main( ) This makes the structure type available to all code that follows the structure definigon To declare two variables of type CDAccount: CDAccount my_account, your_account; my_account and your_account contain disgnct member variables balance, interest_rate, and term 11/29/17 Matni, CS16, Fa17 10

11 Specifying Member Variables Member variables are specific to the structure variable in which they are declared Syntax to specify a member variable (note the. ) Structure_Variable_Name. Member_Variable_Name Given the declaragon: CDAccount my_account, your_account; Use the dot operator to specify a member variable, e.g. my_account.balance my_account.interest_rate is a double is a double my_account.term is an int 11/29/17 Matni, CS16, Fa17 11

12 Note the struct definijon is placed before main() 11/29/17 Matni, CS16, Fa17 12

13 Note the declarajon of CDAccount Note the calculajons done with the structure s member variables 11/29/17 Matni, CS16, Fa17 13

14 Note that the structure is passed into the funcjon as call- by- reference. You can also pass a structure call- by- value. Note the use of the structure s member variables with an input stream. 11/29/17 Matni, CS16, Fa17 14

15 Duplicate Names Member variable names duplicated between structure types are not a problem struct FertilizerStock { double quantity; double nitrogen_content; }; FertilizerStock super_grow; struct CropYield { int quantity; double size; }; CropYield apples; This is because we have to use the dot operator super_grow.quanjty and apples.quanjty are different variables stored in different locagons in computer memory 11/29/17 Matni, CS16, Fa17 15

16 Structures as Return FuncGon Types Structures can also be the type of a value returned by a funcgon Example: CDAccount shrink_wrap (double the_balance, double the_rate, int the_term) { CDAccount temp; } temp.balance = the_balance; temp.interest_rate = the_rate; temp.term = the_term; return temp; What is this funcjon doing? 11/29/17 Matni, CS16, Fa17 16

17 Example: Using FuncGon shrink_wrap shrink_wrap builds a complete structure value in the structure temp, which is returned by the funcgon We can use shrink_wrap to give a variable of type CDAccount a value in this way: CDAccount new_account; new_account = shrink_wrap( , 5.1, 11); 11/29/17 Matni, CS16, Fa17 17

18 Assignment and Structures The assignment operator (=) can also be used to give values to structure types Using the CDAccount structure again for example: CDAccount my_account, your_account; my_account.balance = ; my_account.interest_rate = 5.1; my_account.term = 12; your_account = my_account; Note: This last line assigns all member variables in your_account the corresponding values in my_account 11/29/17 Matni, CS16, Fa17 18

19 Hierarchical Structures Structures can contain member variables that are also structures struct Date { int month; int day; int year; }; struct PersonInfo { double height; int weight; Date birthday; }; struct PersonInfo contains a Date structure 11/29/17 Matni, CS16, Fa17 19

20 Using PersonInfo An example on. operator use A variable of type PersonInfo is declared: PersonInfo person1; To display the birth year of person1, first access the birthday member of person1 cout << person1.birthday (wait! not complete yet!) But we want the year, so we now specify the year member of the birthday member cout << person1.birthday.year; 11/29/17 Matni, CS16, Fa17 20

21 IniGalizing Structures A structure can be inigalized when declared Example: struct Date { int month; int day; int year; }; Can be inigalized in this way watch for the order!: Date due_date = {4, 20, 2018}; Date birthday = {12, 25, 2000}; 11/29/17 Matni, CS16, Fa17 21

22 CLASSES 11/29/17 Matni, CS16, Fa17 22

23 Main Differences: structure vs class Classes in C++ evolved from the concept of structures in C Both classes and structures can have member variables Both classes and structures can have member funcjons, ALTHOUGH classes are made to be easier to use with member funcgons Classes may not be used when interfacing with C, because C does not have a concept of classes (only structures) 11/29/17 Matni, CS16, Fa17 23

24 Example of a Class: DayOfYear DefiniGon class DayOfYear { public: void output( ); int month; int day; }; Member FuncGon DeclaraJon Member Variables DeclaraJon public vs private senngs for members public means these members can be accessed by a program private means they are only for use by the class itself (e.g. test code) 11/29/17 Matni, CS16, Fa17 24

25 Defining a Member FuncGon Member funcgons are declared in the class declaragon Member funcgon defini1ons idengfy class in which the funcgon is a member Note the use of the :: in the following example Member funcgon defini>on syntax: Returned_Type Class_Name::Function_Name(Parameter_List) { Function Body Statements } 11/29/17 Matni, CS16, Fa17 25

26 Defining a Member FuncGon Member funcgon defini>on syntax: Returned_Type Class_Name::Function_Name(Parameter_List) { Function Body Statements } EXAMPLE: void DayOfYear::output() { cout << month = << month <<, day = << day << endl; } 11/29/17 Matni, CS16, Fa17 26

27 The :: Operator :: is called the scope resolu1on operator Indicates what class a member funcgon is a member of Example: void DayOfYear::output( ) indicates that funcgon output is a member of the DayOfYear class The class name that precedes :: is called a type qualifier 11/29/17 Matni, CS16, Fa17 27

28 :: Operator vs.. Operator :: is used with classes to idengfy a member void DayOfYear::output( ) { // function body }. is used with variables to idengfy a member DayOfYear birthday; birthday.output( ); 11/29/17 Matni, CS16, Fa17 28

29 Calling Member FuncGons Calling the DayOfYear member funcgon output: DayOfYear today, birthday; today.output( ); birthday.output( ); Note that today and birthday have their own versions of the month and day variables for use by the output funcgon Also, note how similar this is to other class member funcgons call- outs that we ve done, such as: string Name = Jimbo Jones ; int stlen = Name.length( ); 11/29/17 Matni, CS16, Fa17 29

30 Member Variables/FuncGons Private vs. Public C++ can help us by restricgng the program from directly referencing certain member variables Private members of a class can only be referenced within the definigons of member funcgons and NOT by outside users of the class If the program tries to access a private member, the compiler will give an error message Private is the default senng in classes 11/29/17 Matni, CS16, Fa17 30

31 Public Variables Public variables are the only ones that can be accessed directly by the main program If we want the program to be able to change a class variables values, then they must be declared as public 11/29/17 Matni, CS16, Fa17 31

32 Public or Private Members The keyword private idengfies the members of a class that can be accessed only by member func>ons of the class Members that follow the keyword private are called private members of the class The keyword public idengfies the members of a class that can be accessed from outside the class Members that follow the keyword public are called public members of the class 11/29/17 Matni, CS16, Fa17 32

33 Example class DayOfYear { public: void input(); void output(); private: void check_results(); int var1, var2; }; The member funcgons input( ) and output( ) are accessible from the main( ) or other funcgons. The member funcgon check_results() is strictly to be used internally in DayOfYear class workings, as are int variables var1 and var2. 11/29/17 Matni, CS16, Fa17 33

34 Example from the Textbook: Display 10.4 The program takes in user input on today s date and compares it to J.S. Bach s birthday (i.e. a specific date of 3/21) UGlizes a user- defined class called DayOfYear which holds a date and a month, but ALSO does funcgons like: Input date Check date against set birthday Outputs results 11/29/17 Matni, CS16, Fa17 34

35 The main( ) funcgon int main () { DayOfYear today, bach_birthday; cout << Enter today s date:\n ; today.input(); cout << Today s date is: ; today.output(); bach_birthday.set(3, 21); cout << Bach s Birthday is: ; bach_birthday.output(); if ((today.get_month() == bach_birthday.get_month()) && (today.get_day() == bach_birthday.get_day()) { cout << Happy Birthday, J.S. Bach!!!\n ; } return 0; } Note today & bach_birthday are both objects of the class DayOfYear.input() and.output() are member func>ons of DayOfYear class. Must be public b/c main() is using them..set() is a public member func>on too..get_month() and get_day() are public member func>ons too. What variable types do they look like they return? 11/29/17 Matni, CS16, Fa17

36 DayOfYear Class DefiniGon class DayOfYear { public: void input(); void output(); void set(int newmonth, int newday); int get_month(); int get_day(); private: void check_date(); int month, day; } Q: Why didn t we see the member func>on check_date( ) or the member variables month or day in the main( ) part of the program? A: They re private! 11/29/17 Matni, CS16, Fa17 36

37 void input() { Define All The Member FuncGons input() STOP!!! } 11/29/17 Matni, CS16, Fa17 37

38 Define All The Member FuncGons input() void DayOfYear::input() { cout << Enter the month as a number: ; cin >> month; cout << Enter the day of the month: ; cin >> day; } check_date(); Calling a member func>on! Is this a private or a public one? 11/29/17 Matni, CS16, Fa17 38

39 Define All The Member FuncGons output() void DayOfYear::output() { cout << Month is: ; cout << month << endl; cout << Day of the month is: ; cout << day << endl; } 11/29/17 Matni, CS16, Fa17 39

40 Define All The Member FuncGons set(), get_month() and get_day() void DayOfYear::set(int newmonth, int newday) { month = newmonth; day = newday; check_date(); } int DayOfYear::get_month() { return month; } int DayOfYear::get_day() { return day; } 11/29/17 Matni, CS16, Fa17 40

41 Define All The Member FuncGons check_date() void DayOfYear::check_date() { if ( (month < 1) (month > 12) (day < 1) (day > 31) ) { cout << Illegal date. Aborting program!\n ; exit(1); } } 11/29/17 Matni, CS16, Fa17 41

42 Punng It All Together Check Display 10.4 Example in Textbook for full program. class DayOfYear definigon main() Looks familiar? All the member funcgons of class DayOfYear Same approach with defining funcgons in C++ 11/29/17 Matni, CS16, Fa17 42

43 Using Private Variables It is a pracgce norm to make all member variables private Although, this is not strictly required Private variables require member funcgons to perform all changing and retrieving of values 11/29/17 Matni, CS16, Fa17 43

44 Using Private Variables It is a pracgce norm to make all member variables private FuncGons that allow you to obtain the values of member variables are called accessor funcgons. Example: get_day in class DayOfYear FuncGons that allow you to also change the values of member variables are called mutator funcgons. Example: set in class DayOfYear 11/29/17 Matni, CS16, Fa17 44

45 Review: Declaring an Object Once a class is defined, an object of the class is declared just as variables of any other type This is similar to when you declare a structure in C++ Example: To create two objects of type Bicycle: class Bicycle { // class definition lines };... Bicycle my_bike, your_bike; 11/29/17 Matni, CS16, Fa17 45

46 The Assignment Operator Objects and structures can be assigned values with the assignment operator (=) Example: DayOfYear due_date, tomorrow; tomorrow.set(11, 19); due_date = tomorrow; 11/29/17 Matni, CS16, Fa17 46

47 Review: Calling Public Members Recall that if calling a member funcgon from the main funcgon of a program, you must include the the object name: account1.update( ); Again, just like when we used member funcgons of pre- defined classes, like string 11/29/17 Matni, CS16, Fa17 47

48 Calling Private Members When a member func>on calls a private member funcgon, an object name is not used Example: if fraction (double percent); is a private member of the class BankAccount AND if fraction is called by another member funcgon called update void BankAccount::update( ) { balance = balance + fraction(interest_rate)* balance; } NOT: BankAccount::fraction(interest_rate)*balance; 11/29/17 Matni, CS16, Fa17 48

49 Constructors A constructor can be used to ini>alize member variables when an object is declared A constructor is a member func>on that is usually public and is automagcally called when an object of the class is declared RULE: A constructor s name must be the name of the class A constructor cannot return a value No return type, not even void, is used in declaring or defining a constructor 11/29/17 Matni, CS16, Fa17 49

50 YOUR TO- DOs q Lab 8 due TOMORROW (Wed. 11/29) by noon q HW 9 due Thu. 12/7 q Lab 9 due Wed. 12/6 by noon q Read Ch. 13 on Linked Lists for Thursday q Visit Prof s and TAs office hours if you need help! q Smile! And make people wonder why the heck you re smiling 11/29/17 Matni, CS16, Fa17 50

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